"The increase in version number is proposed due to the sheer volume of work which makes Shiretoko feel like much more than a small, incremental improvement over Firefox 3: TraceMonkey, video tag and player support, improvements to user controls over data privacy, significant improvements in the web layout and rendering platform, and much more," said Mozilla's Mike Beltzner in a blog post last week.

This is probably a good marketing idea and I do think there's enough changes in 3.1 to warrant a higher revision number. I suspect most people would have thought that v3.1 was a very minor upgrade, when in fact there's a lot of significant changes—even if most of them are behind the scene (such as TraceMonkey, Web worker thread support, improvements to the Gecko rendering engine, etc.)